Not applicable.
1. Field of the Invention
The present invention relates to a steel making furnace using electrical current as a heat source and, more particularly, to such a furnace designed and constructed to remain statically positioned through consecutive furnace cycles each cycle being characterized by always maintaining a sufficient large wet heel for flat bath operation through charging and slag free tapping.
2. Description of the Prior Art
It is known in the art of steel making to use electric current as a heat source in a steel making furnace. Arc heating furnaces are used to heat a metal charge by either heat radiation from arcs passed between electrodes above the metal charge or by arcs passing from the electrodes to the metal charge where heat is generated by the electrical resistance of the metal charge. When the furnace has an electrically conductive furnace bottom, the bottom forms part of an electrical circuit powered by direct current. When the furnace has a non-conductive furnace bottom, the electrical circuit is powered by alternating current and the circuit is limited to the electrodes and metal charge. Induction furnaces are also used to heat a metal charge by using either inductors according to a transformer principle where the secondary winding is formed by a loop of liquid metal in a refractory channel or a coreless principle where induction coils surround the furnace wall and generate a magnetic field to impart energy to the metal charge in the furnace.
The present invention is applicable to such electric furnaces and in particular to an alternating current direct arc electric furnace equipped with three electrodes powered by three phase alternating current to establish arcs passed from an electrode to a metal charge to another electrode and from electrode to electrode. The direct-arc electric-furnace as used in the steel industry is primarily a scrap-melting furnace, although molten blast-furnace iron and direct-reduction iron (DRI) are also used for charging the furnace. Combinations of scrap and minor quantities of blast furnace iron or direct reduction iron are common furnace charging compositions. A three-phase transformer, equipped for varying the secondary voltage, is used to supply electrical energy at suitable range of power levels and voltages. Cylindrical solid graphite electrodes are suspended by a mechanism from above the furnace downwardly through ports in a furnace roof to positions so that the electrodes conduct the electric current inside the furnace to maintain arcs for melting and refining a furnace charge. A side wall supports the roof on a lower shell which is provided with a refractory lining to contain the metal charge. The lower shell is pivotally support on a foundation and a furnace tilting drive is operated to tilt the furnace in each of opposite directions for de-slagging and tapping. Other drive mechanisms are provided to remove the roof from the upper shell to gain access to the furnace interior for the introduction of a metal charge.
The tonnage of liquid metal that can be refined in such tilting furnaces is limited by the load bearing capacity of the pivotal support and the furnace tilting drive and the practical limits of the geometry of the hearth. The pivotal support and the tilting drive must take the form of robust structures to sustain and pivot the weight of the entire furnace and its content of liquid steel and slag. The geometry of the hearth when tilting the furnace to tap steel and to clear the tap gate for sand cleaning of the tap hole adds stresses to the pivotal support and tilting drive that increase significantly with an increase to the furnace tilt angle. The tilting of the furnace must be sufficiently slow and carefully controlled to avoid erratic eccentric loading on the tilting mechanism due to the wave like shock loading as the liquid steel shifts back and forth in the volume of the hearth of the furnace. The drive mechanism and support structure to tilt the electric furnace represents a significant capital expenditure. Costs are also incurred by the required maintenance to prevent a serious consequence should the tilt structure fail to allow draining of the heat from a furnace. The practical limits of the geometry of a tilting furnace hearth limit the depth of steel above the tap hole and therefore limit the maximum diameter of the tap hole that can be used and still have slag free tapping. This small size tap hole results in longer tapping times. Draining most of the steel from the furnace prolongs the time between tapping of the furnace because of the need to reestablish a liquid metal bath using significant quantities of electric power for the heating the metal charge. It is known in the art to retain a quantity of the steel in the furnace after tapping which is commonly called a wet heel practice. However, the structural integrity of the furnace mandates that the slag line be inspected periodically, typically every three to twelves heats with repairs performed based on the slag line condition. Generally, gunning will be performed several times a week. Periodically, every two-three weeks, the complete furnace bottom will be exchanged with a newly rebuilt bottom and worn bottom will have its side walls in the slag line area undergo a major repair.
Accordingly, it is an object of the present invention to provide an electric furnace suitable for use in a green field installation, to revamp existing installations to form a steel making facility for supply of ladles of steel at temperatures and tonnages significantly greater than provided by known electrically heated furnaces.
It is another object of the present invention relates to a steel making method and furnace construction to improve electric furnace operating efficiency and steel making capacity of an electric furnace.
It is a further object of the present invention to provide a versatile electric furnace design to simplify furnace maintenance and to maintain a large liquid metal hot heel for promoting flat bath operation and slag free tapping of a heat.
According to the present invention there is provided an electric furnace for steel making, the furnace including the combination of a lower furnace shell stationarily supported during charging, heating and tapping of a heat, the lower shell having a floor wall with a sloping contour to increase a liquid metal depth of a heat to at least three times the diameter of a tap hole at a site communicating with the tap hole for slag free tapping of a heat, the lower furnace shell having a liquid metal capacity to maintain a liquid metal heel of at least 70% of a heat before tapping for flat bath refining of a heat throughout the charging and heating of a heat, an upper furnace shell supported by the lower furnace shell, a furnace roof supported by the upper furnace shell, an electrically powered member for heating a metal charge in the lower furnace shell, and a control including plugging for the tap hole to control tapping of a heat form the lower furnace shell. A lower shell stationarily supported during furnace operations consisting of charging, heating and tapping of a heat, the lower shell having a floor with a sloping contour to increase liquid metal depth at a site communicating with a tap hole for tapping of a heat, an upper shell supported by the lower shell, a roof supported by the upper shell, the roof including at least one aperture for passage of an electrode to heat a metal charge in the lower, shell, an electrode positioned by electrode carrier arm relative to the aperture for heating a metal charge in the lower shell, and a plug member operatively associated with the tap hole for maintaining a liquid hot heel in the lower shell after tapping of a heat.
Accordingly, the present invention also provides a method for producing steel in an electric furnace, the method of including the steps of providing an electric furnace including a furnace shell having a sloping floor extending downwardly to a tap hole refining a steel melt in the furnace using electric current to form a first heat, tapping a sufficient quantity of steel from the first heat to a ladle while the lower shell remains stationary to maintain a liquid hot heel in the furnace consisting of at least 70% of the tapped steel, maintaining flat bath furnace operation by using electric current and latent heat of the liquid hot heel to refine charged material in the furnace for forming a second heat, and tapping a sufficient quantity of steel from the second heat while the lower shell remains stationary to maintain a liquid hot heel in the furnace consisting of at least 70% of the tapped steel.